U.S. patent application number 11/788239 was filed with the patent office on 2008-01-17 for secret communications system and channel control method.
This patent application is currently assigned to NEC Corporation. Invention is credited to Wakako Maeda, Akio Tajima, Seigo Takahashi, Akihiro Tanaka.
Application Number | 20080013738 11/788239 |
Document ID | / |
Family ID | 38326251 |
Filed Date | 2008-01-17 |
United States Patent
Application |
20080013738 |
Kind Code |
A1 |
Tajima; Akio ; et
al. |
January 17, 2008 |
Secret communications system and channel control method
Abstract
A secret communications system realizes point-to-multipoint or
multipoint-to-multipoint connections of both quantum channels and
classical channels. Multiple remote nodes are individually
connected to a center node through optical fiber, and random-number
strings K1 to KN are individually generated and shared between the
respective remote nodes and the center node. Encrypted
communication is performed between each remote node and the center
node by using the corresponding one of the shared random-number
strings K1 to KN as a cryptographic key. The center node is
provided with a switch section for quantum channels and a switch
section for classical channels. Switching control on each of these
switch sections is performed independently of the other by a
controller.
Inventors: |
Tajima; Akio; (Tokyo,
JP) ; Tanaka; Akihiro; (Tokyo, JP) ; Maeda;
Wakako; (Tokyo, JP) ; Takahashi; Seigo;
(Tokyo, JP) |
Correspondence
Address: |
SCULLY SCOTT MURPHY & PRESSER, PC
400 GARDEN CITY PLAZA
SUITE 300
GARDEN CITY
NY
11530
US
|
Assignee: |
NEC Corporation
Tokyo
JP
|
Family ID: |
38326251 |
Appl. No.: |
11/788239 |
Filed: |
April 19, 2007 |
Current U.S.
Class: |
380/278 ;
380/44 |
Current CPC
Class: |
H04L 9/0852 20130101;
Y04S 40/20 20130101; H04L 9/0855 20130101 |
Class at
Publication: |
380/278 ;
380/044 |
International
Class: |
H04L 9/08 20060101
H04L009/08; H04L 9/28 20060101 H04L009/28 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 19, 2006 |
JP |
2006-116097 |
Claims
1. A secret communications system comprising: a center node; and a
plurality of remote nodes, each of which is connected to the center
node through an optical transmission line, wherein a plurality of
channels are set between the center node and each remote node,
wherein the center node comprises a switch for independently
switching each of the channels corresponding to each remote node
between the remote nodes such that the channels are used to perform
processing of: generation of shared random number to be used as
cryptographic keys; and cryptographic communication using a
cryptographic key.
2. The secret communications system according to claim 1, wherein
the switch is controlled such that a channel used for the
processing of generation of shared random number is sequentially
switched between the remote nodes.
3. The secret communications system according to claim 1, wherein
the switch is controlled based on an amount of the shared random
numbers for each remote node such that a channel used for the
processing of generation of shared random number is sequentially
switched between the remote nodes.
4. The secret communications system according to claim 3, wherein
the switch is controlled based on a generation rate of the shared
random numbers for each remote node.
5. The secret communications system according to claim 3, wherein
the switch is controlled based on a consumption rate of the shared
random numbers for each remote node.
6. The secret communications system according to claim 1, wherein
the processing of generation of shared random number is performed
by a quantum key distribution technique.
7. The secret communications system according to claim 6, wherein
the shared random number is generated by a plug-and-play quantum
key distribution system.
8. The secret communications system according to claim 6, wherein
the shared random number is generated by a one-way quantum key
distribution system.
9. The secret communications system according to claim 1, wherein
the cryptographic communication is performed based on one-time pad
cryptography using a cryptographic key generated from the shared
random number.
10. The secret communications system according to claim 1, wherein
the cryptographic communication is performed based on block key
cryptography using a cryptographic key generated from the shared
random number.
11. A secret communication device connected to each of a plurality
of remote nodes through an optical transmission line, wherein a
plurality of channels are set with each remote node, comprising: a
plurality of switches, each of which is provided for each of the
channels corresponding to each remote node and switches between the
remote nodes; and a controller for independently controlling the
plurality of switches such that the channels are used to perform
processing of: generation of shared random number to be used as
cryptographic keys; and cryptographic communication using a
cryptographic key.
12. The secret communication device according to claim 11, wherein
the controller controls the plurality of switches based on an
amount of the shared random numbers for each remote node such that
a channel used for the processing of generation of shared random
number is sequentially switched.
13. The secret communication device according to claim 12, wherein
the switch is controlled based on a generation rate of the shared
random numbers for each remote node.
14. The secret communication device according to claim 12, wherein
the switch is controlled based on a consumption rate of the shared
random numbers for each remote node.
15. The secret communication device according to claim 11, wherein
the cryptographic communication is performed based on one-time pad
cryptography using a cryptographic key generated from the shared
random number.
16. The secret communication device according to claim 11, wherein
the cryptographic communication is performed based on block key
cryptography using a cryptographic key generated from the shared
random number.
17. A channel control method for a secret communication device
connected to each of a plurality of remote nodes through an optical
transmission line, wherein a plurality of channels are set with
each remote node, comprising: independently controlling a plurality
of switches, each of which is provided for each of the channels
corresponding to each remote node, to switch between the remote
nodes in order to use the channels to perform processing of:
generation of shared random number to be used as cryptographic keys
and cryptographic communication using a cryptographic key.
18. The channel control method according to claim 17, further
comprising: monitoring an amount of the shared random numbers for
each remote node; and sequentially switching a channel used for the
processing of generation of shared random number based on the
amount of the shared random numbers for each remote node.
19. The channel control method according to claim 17, wherein the
cryptographic communication is performed between the plurality of
remote nodes by setting a common cryptographic key on the plurality
of remote nodes.
20. A program implementing a secret communication device on a
computer, wherein the secret communication device is connected to
each of a plurality of remote nodes through an optical transmission
line, wherein a plurality of channels are set with each remote
node, the program comprising: independently controlling a plurality
of switches, each of which is provided for each of the channels
corresponding to each remote node, to switch between the remote
nodes in order to use the channels to perform processing of:
generation of shared random number to be used as cryptographic keys
and cryptographic communication using a cryptographic key.
21. The program according to claim 20, further comprising:
monitoring an amount of the shared random numbers for each remote
node; and sequentially switching a channel used for the processing
of generation of shared random number based on the amount of the
shared random numbers for each remote node.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a secret communications
system and, more particularly, to a secret communications system,
secret communication apparatus, and channel control method,
enabling point-to-multipoint and/or multipoint-to-multipoint
cryptographic key sharing and encrypted communication.
[0003] 2. Description of the Related Art
[0004] In recent years, the internet has been economic and social
infrastructure over which various data are exchanged. Therefore, it
is an important issue to devise preventive measures to protect the
data flowing over networks from risks of eavesdropping. One of the
preventive measures is a secret communications system by which data
for communication is encrypted. For the encryption method, there
are two kinds of schemes: private key cryptography and public key
cryptography.
[0005] The private key cryptography, as is typified by AES
(Advanced Encryption Standard), is a scheme using a common
cryptographic key for both encryption and decryption, and is
capable of high-speed processing. For this reason, this scheme is
used to encrypt data itself.
[0006] On the other hand, the public key cryptography, as is
typified by RSA (Rivest Shamir Adleman), is a scheme based on a
one-way function, using a public key for encryption and a secret
key for decryption. Since this scheme is not suitable for
high-speed processing, it is used to distribute a cryptographic key
for the private key scheme.
[0007] In secret communications where secrecy is ensured by
encrypting data, an important thing to ensure the secrecy is that
encrypted data cannot be broken even if the encrypted data is
intercepted by an eavesdropper. To do so, it is necessary not to
consecutively use the same key for encryption. This is because the
consecutive use of the same key for encryption may increase the
possibility that the encryption key is estimated based on the
increased amount of intercepted data.
[0008] Accordingly, it is required to update a cryptographic key
shared between a sender and a receiver. It is indispensable that
the key being updated is not intercepted and broken during key
update. Therefore, to update the key, there are two broad types of
methods: (1) a method in which the key is encrypted for
transmission through the public key cryptography, and (2) a method
in which the key is encrypted for transmission by using a master
key that is a common key preset for key update. (For example, see
Japanese Patent Application Unexamined Publication Nos. 2002-344438
and 2002-300158.) The security according to any of these methods
depends on the fact that an enormous amount of calculation is
required for cryptanalysis.
[0009] On the other hand, quantum key distribution (QKD)
technology, unlike ordinary (classical) optical communications, is
a technology that allows a sender and a receiver to generate and
share a cryptographic key by the transmission of a single photon
per bit. See the following papers: [0010] Bennett, C. H., and
Brassard, G., "Quantum cryptography: Public key distribution and
coin tossing" in Proceedings of IEEE International Conference on
Computers, Systems, and Signal Processing, Bangalore, India, 10-12
Dec. 1984, pp. 175-179; and [0011] Ribordy, G., Gautier, J.-D.,
Gisin, N., Guinnard, O., and Zbinden, H., "Automated `plug &
play` quantum key distribution," Electronics Letters, Vol. 34, No.
22 (1998), pp. 2116-2117)
[0012] According to this QKD technology, unlike the conventional
technologies, the security does not depend on the amount of
calculation, but the impossibility of eavesdropping has been proved
on the basis of quantum mechanics. Therefore, since the security of
the photon-transmission portion of a system can be ensured by
virtue of this technology, not only point-to-point key generation
and sharing but also point-to-multipoint, or
multipoint-to-multipoint, key generation and sharing can be
achieved by using the techniques of optical switching and passive
optical splitting (see Townsend, P. D., "Quantum cryptography on
multi-user optical fibre networks," Nature, Vol. 385, 2 Jan. 1997,
pp. 47-49).
[0013] As mentioned above, when a shared cryptographic key is
updated, the security is based on the fact that an enormous amount
of calculation is required for cryptanalysis, in each of the method
of sending the updated key after encrypting it through the public
key cryptography and the method of sending the updated key after
encrypting it by using a common key-preset for update. Therefore,
there has been a problem that the secrecy is degraded with
improvements in cryptanalysis technology, such as an improvement in
computer performance and the advent of an evolved cryptanalysis
algorithm. For example, in the 56-bit DES challenge contests where
contestants compete in time to break DES (Data Encryption
Standard), which is a common key cipher, although it took 96 days
to break DES in 1997, the time was reduced to 22 hours in 1999. As
for a public key cipher, although it took eight months to break a
RSA public key cipher with a key length of 429 bits in 1994, it
took about three months to break one with a key length of 576 bits
in 2004. As described above, the cryptanalysis technology has been
improving.
[0014] In the quantum key distribution (QKD) technology, to
accomplish an extension to the point-to-multipoint or
multipoint-to-multipoint key generation and sharing by using the
techniques of optical switching and passive optical splitting, it
is necessary to realize not only point-to-multipoint or
multipoint-to-multipoint connections of photon transmission
(quantum channels) but also point-to-multipoint or
multipoint-to-multipoint connections of classical channels to carry
out key generation and sharing based on the result of photon
transmission, as well as encrypted communication.
[0015] However, according to the technologies to date, only
point-to-multipoint connections of the quantum-channel portion has
been realized. In order to realize point-to-multipoint or
multipoint-to-multipoint connections of both quantum channels and
classical channels, consideration should be given to the fact that
there is a great difference between the rate of a quantum channel
(photon transmission rate) and the communication rate of a
classical channel for key generation and encrypted communication.
That is, the quantum channel and the classical channel are
different communications, and therefore it is necessary to satisfy
the condition that the switching of quantum-channel connections and
the switching of classical-channel connections be performed at
different timings. The hitherto technologies could not satisfy such
a condition.
[0016] In addition, if a network is built by using different fibers
for quantum channels and classical channels respectively, the
problems arise not only that the cost of fiber laying increases but
also that an action of eavesdropping on the classical channel fiber
cannot be detected.
SUMMARY OF THE INVENTION
[0017] To solve the above-described problems, in a secret
communications system according to the present invention, random
numbers are generated and shared between a center node and each of
multiple remote nodes. An encrypted communication is carried out by
using the random numbers as a cryptographic key. Channels for the
random-number generation and sharing and channels for the encrypted
communication are independently switched.
[0018] According to an aspect of the present invention, a secret
communications system includes a center node and a plurality of
remote nodes, each of which is connected to the center node through
an optical transmission line, wherein a plurality of channels are
set between the center node and each remote node. The center node
includes a switch for independently switching each of the channels
corresponding to each remote node between the remote nodes such
that the channels are used to perform processing of: generation of
shared random number to be used as cryptographic keys; and
cryptographic communication using a cryptographic key.
[0019] According to an embodiment of the present invention, the
switch is controlled such that a channel used for the processing of
generation of shared random number is sequentially switched between
the remote nodes. Preferably, the switch is controlled based on an
amount of the shared random numbers for each remote node. In this
case, the switch may be controlled based on a generation rate of
the shared random numbers for each remote node or based on a
consumption rate of the shared random numbers for each remote
node.
[0020] As an example, the processing of generation of shared random
number is performed by a quantum key distribution technique. The
shared random number may be generated by a plug-and-play quantum
key distribution system or a one-way quantum key distribution
system. The cryptographic communication may be performed based on
one-time pad cryptography or block key cryptography using a
cryptographic key generated from the shared random number.
[0021] According to another aspect of the present invention, a
secret communication device connected to each of a plurality of
remote nodes through an optical transmission line, wherein a
plurality of channels are set with each remote node, includes: a
plurality of switches, each of which is provided for each of the
channels corresponding to each remote node and switches between the
remote nodes; and a controller for independently controlling the
plurality of switches such that the channels are used to perform
processing of: generation of shared random number to be used as
cryptographic keys; and cryptographic communication using a
cryptographic key.
[0022] According to still another aspect of the present invention,
a channel control method for a secret communication device
connected to each of a plurality of remote nodes through an optical
transmission line, wherein a plurality of channels are set with
each remote node, including: independently controlling a plurality
of switches, each of which is provided for each of the channels
corresponding to each remote node, to switch between the remote
nodes in order to use the channels to perform processing of:
generation of shared random number to be used as cryptographic keys
and cryptographic communication using a cryptographic key. The
cryptographic communication may be performed between the plurality
of remote nodes by setting a common cryptographic key on the
plurality of remote nodes.
[0023] As described above, according to the present invention, a
plurality of channels are independently switched between a
plurality of remote nodes and the processing of shared random
number generation and encrypted communications are effectively
performed using the channels. Accordingly, in a network
configuration with a small number of laid optical fibers, it is
possible to realize efficient photon transmission, quantum key
generation and sharing, and encrypted communication in
point-to-multipoint or multipoint-to-multipoint connections.
[0024] Further, according to the present invention, it is possible
to realize the quantum encryption key generation and encrypted
communication using the quantum encryption key in a
point-to-multipoint or multipoint-to-multipoint connection
system.
[0025] In addition, the switching control is performed while
monitoring the amount of random numbers, ensuring the amount of
encryption key and the stability of encrypted communication at all
times.
[0026] Applying the present invention to a quantum key distribution
system, the switching of remote nodes to connect through a quantum
channel and the switching of remote nodes to connect through a
classical channel are separately handled by the optical switches
and thus performed independently, whereby it is possible to realize
efficient photon transmission, quantum key generation and sharing,
and encrypted communication in the point-to-multipoint connection.
In addition, by multiplexing and transmitting the quantum and
classical channels over a single fiber, it is possible to construct
a system at a low cost for fiber laying.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 is a block diagram showing a schematic configuration
of a secret communications system according to a first mode of the
present invention.
[0028] FIG. 2 is a block diagram showing a key generation function
of a center node in the first mode of the present invention.
[0029] FIG. 3 is a time chart showing an example of (a) switching
control of quantum channels and (b) switching control of classical
channels by the center node in the first mode.
[0030] FIG. 4 is a block diagram showing a schematic configuration
of a secret communications system according to a first embodiment
of the present invention.
[0031] FIG. 5A is a block diagram showing an example of a quantum
transmitter on Alice's side (remote-node side) in a plug and play
QKD system.
[0032] FIG. 5B is a block diagram showing an example of a quantum
receiver on Bob's side (center-node side) in the plug and play QKD
system.
[0033] FIG. 6A is a schematic diagram showing an example of an
optical switch.
[0034] FIG. 6B is a schematic diagram showing another example of
the optical switch.
[0035] FIG. 7 is a block diagram showing a schematic configuration
of a secret communications system according to a second embodiment
of the present invention.
[0036] FIG. 8A is a block diagram showing an example of a quantum
transmitter on Alice's side (remote-node side) in a one-way QKD
system.
[0037] FIG. 8B is a block diagram showing an example of a quantum
receiver on Bob's side (center-node side) in the one-way QKD
system.
[0038] FIG. 9 is a block diagram showing a schematic configuration
of a secret communications system according to a third embodiment
of the present invention.
[0039] FIG. 10A is a block diagram showing another example of the
quantum transmitter on Alice's side (remote-node side) in a plug
and play QKD system.
[0040] FIG. 10B is a block diagram showing another example of the
quantum receiver on Bob's side (center-node side) in the plug and
play QKD system.
[0041] FIG. 11A is a block diagram showing another example of the
quantum transmitter on Alice's side (remote-node side) in a one-way
QKD system.
[0042] FIG. 11B is a block diagram showing another example of the
quantum receiver on Bob's side (center-node side) in the one-way
QKD system.
[0043] FIG. 12 is a block diagram showing a schematic configuration
of a secret communications system according to a second mode of the
present invention.
[0044] FIG. 13 is a block diagram showing a schematic configuration
of a secret communications system according to a fourth embodiment
of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
1. First Mode
1.1) System Configuration
[0045] FIG. 1 is a block diagram showing a schematic configuration
of a secret communications system according to a first mode of the
present invention. Here, each of N (multiple) remote nodes 1 to N
is connected to a center node 10 through optical fiber, and
generation and sharing of a cryptographic key, as well as encrypted
communication using the cryptographic key, are performed between
the center node 10 and each remote node.
[0046] Each of the remote nodes 1 to N has substantially the same
configuration and includes a quantum-channel unit 21, a
classical-channel unit 22, a controller 23 for controlling these
units, and a key memory 24 for storing a string of random numbers
to be used for a cryptographic key. The respective key memories 24
of the remote nodes 1 to N store random-number strings K1, K2, . .
. , KN, respectively, that are individually generated and shared
with the center node 10. The controller 23 executes with the center
node 10 the generation of the shared random-number string,
encryption/decryption using the shared random-number string, and
the like, which will be described later. The controller 23 may be a
program-controlled processor, which can implement the
above-mentioned random-number generation function and
encryption/decryption function by executing programs read out of a
memory (not shown).
[0047] The center node 10 includes a switch section 101 and a unit
102 for quantum channels, a switch section 103 and a unit 104 for
classical channels, a controller 105 for controlling these sections
and units, and a key memory 106 for storing the random-number
strings K1, K2, . . . , KN shared with the remote nodes 1 to N,
respectively. Individually with each remote node, the controller
105 executes the generation of the shared random-number string,
switching control on the switch sections 101 and 103,
encryption/decryption using the shared random-number string,
monitoring of the amount of each key (random-number string) stored
in the key memory 106, and the like, which will be described later.
The controller 105 of the center node 10 in particular can control
the switch section 101 for quantum channels and the switch section
103 for classical channels independently.
[0048] The quantum-channel unit 21 of each remote node and the
quantum-channel unit 102 of the center node 10 generate a
random-number string to be shared between them by transmitting a
very weak optical signal through the switch section 101 and a
quantum channel. The classical-channel unit 22 of each remote node
and the classical-channel unit 104 of the center node 10 exchange
data for generating the shared random-number string with each other
through the switch section 103 and a classical channel, and also
exchange data encrypted based on the shared random-number string
with each other through the switch section 103 and the classical
channel.
[0049] The controller 105 controls the switch section 101 and
thereby can connect the quantum channel corresponding to a selected
one of the remote nodes 1 to N to the quantum-channel unit 102.
Independently of this switching control of quantum channels, the
controller 105 controls the switch section 103 and thereby can
connect the classical channel corresponding to a selected one of
the remote nodes 1 to N to the classical-channel unit 104. Since
the switching control of quantum channels and the switching control
of classical channels can be performed independently as described
above, it is possible to efficiently carry out point-to-multipoint
and/or multipoint-to-multipoint photon transmission, communication
for cryptographic-key generation, and communication of encrypted
data, which will be described in detail later.
[0050] Incidentally, as to the channels, it suffices that each of
the quantum and classical channels can be discriminated as a
channel from each other. The quantum channel is a channel used to
generate a quantum key, and the classical channel is a channel for
communications in the range of usual optical power. The classical
channel is used to transmit data for generating the shared
random-number string as well as to transmit encrypted data. The
quantum channel transmits from a sender (Alice) to a receiver (Bob)
an optical signal in a very weak power state where the power is
equivalent to one photon per bit or lower, but also can transmit an
optical signal with the optical power that is used for usual
optical communication.
[0051] Additionally, in the present mode, a quantum channel and a
classical channel are multiplexed. However, the scheme for
multiplexing the channels is not particularly specified. In the
case of wavelength division multiplexing, a wavelength
multiplexer/demultiplexer is provided to the center node 10, at the
stage previous to the switch sections 101 and 103, correspondingly
to each remote node so that a quantum-channel-wavelength signal and
a classical-channel-wavelength signal are demultiplexed and
outputted to the switch sections 101 and 103, respectively.
1.2) Center Node
[0052] FIG. 2 is a block diagram showing the key generation
function of the controller of the center node in the first mode of
the present invention. The controller 105 of the center node 10
controls the entire operation of the center node 10. However,
particularly speaking of the key generation function according to
the present mode, the controller 105 includes a key amount monitor
107 for monitoring the amounts of the keys for the respective
remote nodes, stored in the key memory 106, and a key generation
controller 108 for generating a random-number string shared with
each remote node. The key generation controller 108 controls the
switch sections 101 and 103, based on the amounts of the keys
monitored by the key amount monitor 107. Note that the controller
105 may also be a program-controlled processor, which can implement
the functions equivalent to the key amount monitor 107 and key
generation controller 108, as well as the function of switching the
switch sections 101 and 103 and the encryption/decryption function,
by executing a program read out of a memory (not shown).
[0053] The key generation controller 108 executes a predetermined
sequence for key generation and thereby shares a random-number
string for a cryptographic key with each remote node. As a typical
example, the key generation controller 108 carries out the BB84
protocol (see Bennett and Brassard), error detection/correction,
and privacy amplification, thereby generating and sharing a
cryptographic key. As an example, description will be given below
of the case of generating the random-number string K1 to be shared
with the remote node 1.
[0054] First, for example, when the key amount monitor 107 detects
that the remaining amount of the random-number string K1 has been
reduced, the key generation controller 108 controls the switch
section 101 and thereby connects the quantum channel corresponding
to the remote node 1 to the quantum-channel unit 102. After
establishing synchronization for operation timing with the remote
node 1, the key generation controller 108 receives a very weak
optical signal from the remote node 1 through the quantum channel.
Subsequently, the key generation controller 108 controls the switch
section 103 and thereby connects the classical channel
corresponding to the remote node 1 to the classical-channel unit
104. Then, based on the data obtained by detecting the very weak
optical signal received through the quantum channel, the key
generation controller 108 generates the shared random-number string
K1 by carrying out basis reconciliation, error
detection/correction, and privacy amplification, and stores the
random-number string K1, while relating it to the remote node 1, in
the key memory 106.
[0055] The random-number strings K2 to KN to be shared with the
other remote nodes 2 to N, respectively, are also sequentially
generated through similar processes and then stored in the key
memory 106. The key generation controller 108 may sequentially
select each of the remote nodes 1 to N in this order by controlling
the switch section 101. Alternatively, the key generation
controller 108 may also select a remote node for which a key needs
to be generated, based on the detection result obtained by allowing
the key amount monitor 107 to detect the remaining amounts of the
keys, key generation rates, or key consumption rates.
1.3) Switching Control on Switch Section
[0056] FIG. 3 is a time chart showing an example of (a) switching
control of quantum channels and (b) switching control of classical
channels by the center node in the first mode. Referring to FIG.
3(a), by controlling the switch section 101, the key generation
controller 108 sequentially switches remote nodes to connect to the
quantum-channel unit 102 in the order of the remote node 1 (Alice
1), remote node 2 (Alice 2), . . . , and remote node N (Alice N),
and receives a very weak optical signal from each remote node.
[0057] Referring to FIG. 3(b), in parallel with the switching
control on the switch section 101, while controlling the switch
section 103 and thereby selecting a remote node to connect, the key
generation controller 108 can allow the classical-channel unit 104
to carry out key generation processes such as basis reconciliation
or to exchange encrypted data with the classical-channel unit on
the remote-node side.
[0058] In the example shown in FIG. 3, a very weak optical signal
is first received from the remote node 1 (Alice 1) through the
corresponding quantum channel as shown in FIG. 3(a). Then, as shown
in FIG. 3(b), based on the data obtained by detecting the very weak
optical signal, the key generation processes are carried out with
the classical-channel unit 22 of the remote node 1 through the
corresponding classical channel, and the shared random-number
string K1 is stored in the key memory 106. Data for transmission is
encrypted by using a cryptographic key extracted from this shared
random-number string K1, whereby an encrypted data communication
can be performed between the remote node 1 (Alice 1) and the center
node 10 (Bob) as shown in FIG. 3(b).
[0059] Subsequently, when a very weak optical signal is received
from the remote node 2 (Alice 2) through the corresponding quantum
channel as shown in FIG. 3(a), then, based on the data obtained by
detecting the very weak optical signal, the key generation
processes are carried out with the classical-channel unit 22 of the
remote node 2 through the corresponding classical channel as shown
in FIG. 3(b). It is also possible to interrupt the key generation
processes. In this example, in the middle of the key generation
processes with the remote node 2 (Alice 2), the data generated up
to then are stored, the switch section 103 is switched to the
remote node 1 (Alice 1), and the encrypted data communication with
the remote node 1 (Alice 1) is resumed. When the encrypted data
communication with the remote node 1 (Alice 1) is finished, the
interrupted key generation processes with the remote node 2 (Alice
2) is resumed, and the shared random-number string K2 is stored in
the key memory 106. Data for transmission is encrypted by using a
cryptographic key extracted from this shared random-number string
K2, whereby an encrypted data communication can be performed
between the remote node 2 (Alice 2) and the center node 10 (Bob) as
shown in FIG. 3(b). Thereafter, the operation similarly
continues.
[0060] As described above, the switching of nodes to connect
through a quantum channel and the switching of nodes to connect
through a classical channel are performed independently by using
the switch sections 101 and 103, respectively, whereby it is
possible to flexibly schedule the key generation processes and
encrypted data communications. Accordingly, it is possible to
realize efficient point-to-multipoint photon transmission, quantum
key generation and sharing, and encrypted communication. In
addition, by multiplexing and transmitting the quantum and
classical channels over a single fiber, it is possible to construct
a system at a low cost for fiber laying.
1.4) First Embodiment
[0061] FIG. 4 is a block diagram showing a schematic configuration
of a secret communications system according to a first embodiment
of the present invention. Here, remote nodes 100-1 to 100-N are
individually connected to a center node 200 through optical fiber
transmission lines 300-1 to 300-N, respectively.
[0062] Each remote node 100-i (i=1, 2, . . . , N) includes a key
generator (Key Gen.) 110-i, a quantum transmitter (QTx) 120-i, an
optical multiplexer/demultiplexer 130-i, a classical transceiver
(CTRx) 140-i, an encoder/decoder (Encode/Decode) 150-i, and a key
memory 160-i. The key memory 160-i stores a shared random-number
string Ki generated between its own remote node 100-i and the
center node 200.
[0063] The center node 200 includes a key generator (Key Gen.) 210,
a quantum receiver (Q_Rx) 220, optical multiplexers/demultiplexers
230-1 to 230-N, a classical transceiver (CTRx) 240, an
encoder/decoder (Encode/Decode) 250, a key memory 260, optical
switches 271 and 272, and optical switch controllers 280 and 290.
The key memory 260 stores the shared random-number strings K1 to KN
corresponding to the remote nodes 100-1 to 100-N, respectively.
[0064] Incidentally, in the present embodiment, a quantum signal
and a classical signal use mutually different wavelengths.
Wavelength multiplexing and demultiplexing of quantum channels
(broken lines) and classical channels (solid lines) are performed
by the optical multiplexers/demultiplexers 130-1 to 130-N and 230-1
to 230-N. Signals are wavelength-multiplexed and transmitted over
the optical fibers 300-1 to 300-N. Each optical
multiplexer/demultiplexer 230-i connected to its corresponding
optical fiber transmission line 300-i connects the quantum channel
to the optical switch 271 and connects the classical channel to the
optical switch 272.
[0065] The quantum transmitter 120-i of each remote node 100-i is
sequentially connected to the quantum receiver 220 of the center
node 200 by the optical switch 271, as shown in the time chart of
FIG. 3(a), and photon transmission is carried out between the
connected quantum transmitter and receiver. Here, the switching is
performed in the order of the remote nodes 100-1, 100-2, . . . ,
and 100-N. The switching control on the optical switch 271 is
performed by the optical switch controller 280, based on the
amounts of the keys in the key memory 260, individually generated
and shared with the respective remote nodes.
[0066] On the other hand, the classical transceiver 140-i of each
remote node 100-i is sequentially connected to the classical
transceiver 240 of the center node 200 by the optical switch 272,
and then classical-channel communications are carried out. In the
present embodiment, quantum key generation and sharing based on the
result of photon transmission, and encrypted communication using
the quantum cryptographic key are carried out through a classical
channel. The quantum key generation is carried out between the key
generator 110-i and the key generator 210 through the classical
channel, and the generated key Ki is stored in each of the key
memory 160-i and the key memory 260.
[0067] The key generator 260 of the center node 200 stores and
manages the keys generated for the remote nodes respectively. In an
encrypted communication, for example, one-time pad encryption is
performed by using the key Ki generated for each remote node. In
the case of the one-time pad encryption, a key is discarded each
time an encrypted communication (encoding and decoding) is carried
out. Accordingly, the key in the key memory 260 is consumed
depending on the amount of an encrypted communication. Therefore,
the amounts of the keys in the key memory 260, generated and shared
with the remote nodes respectively, are each monitored, and the
switch controller 280 performs the switching control on the optical
switch 271, based on the amounts of the keys.
[0068] Switching between the quantum key generation and sharing and
the encrypted communication is performed by the classical
transceiver 240. The control on the optical switch 272 to switch
nodes to connect is performed by the optical switch controller 290.
The switching control of classical channels is performed
independently of the switching control of quantum channels, as
shown in the time chart of FIG. 3(b).
[0069] As described above, the switching of nodes to connect
through a quantum channel and the switching of nodes to connect
through a classical channel are separately handled by the optical
switches 271 and 272 and thus performed independently, whereby it
is possible to realize efficient photon transmission, quantum key
generation and sharing, and encrypted communication in the
point-to-multipoint connection. In addition, by multiplexing and
transmitting the quantum and classical channels over a single
fiber, it is possible to construct a system at a low cost for fiber
laying.
Example I
[0070] Next, a specific example will be described in which the
above-described first embodiment is applied to a plug and play
quantum key distribution (QKD) system.
[0071] FIG. 5A is a block diagram showing an example of a quantum
transmitter on Alice's side (remote-node side) in a plug and play
QKD system. FIG. 5B is a block diagram showing an example of a
quantum receiver on Bob's side (center-node side) in the plug and
play QKD system. The quantum transmitter 30 and the quantum
receiver 40 shown in FIGS. 5A and 5B respectively are of the
alternative-shifted phase modulation, plug and play type (see
Ribordy et al., as well as Tanaka, A., Tomita, A., Tajima, A.,
Takeuchi, T., Takahashi, S., and Nambu, Y., "Temperature
independent QKD system using alternative-shifted phase modulation
method" in Proceedings of European Conference on Optical
Communication (2004), Tu4.5.3).
[0072] In this example, the quantum transmitter 30 includes a
polarization beam splitter (PBS) 31, a phase modulator (PMA) 32, a
random number generator (Rnd.) 33, a synchronization section (Sync)
34, and an optical multiplexer/demultiplexer 35. The quantum
transmitter 30 is connected to an optical fiber transmission line
300. The quantum transmitter 30 has an PBS loop composed of the
phase modulator 32 and the polarization beam splitter 31. The PBS
loop has a function similar to a Faraday mirror and outputs
incident light with its polarization state rotated by 90 degrees
(see Tanaka et al.).
[0073] The phase modulator 32 performs phase modulation on a train
of passing optical pulses in accordance with a clock signal
supplied from the synchronization section 34. There are four phase
modulation depths (0, .pi./2, .pi., 3.pi./2) corresponding to the
four combinations of two random number values of two random-number
strings RND0 and RND1 supplied from the random number generator 22.
A phase modulation is performed at the timing when an optical pulse
is passing through the phase modulator 32.
[0074] The quantum receiver 40 includes a polarization beam
splitter (PBS) 41, a phase modulator (PM.sub.B) 42, a random number
generator (Rnd.) 43, a synchronization section (Sync) 44, an
optical multiplexer/demultiplexer 45, an optical coupler 46, an
optical circulator 47, a photon detector section 48, and a pulse
light source 49. The quantum receiver 40 is connected to the
optical fiber transmission line 300. An optical pulse P, generated
by the pulse light source 49 in accordance with a clock signal
supplied from the synchronization section 44, is led by the optical
circulator 47 into the optical coupler 46, where the optical pulse
P is split into two pulses. One of the split optical pulses, an
optical pulse P1, is allowed along a short path and sent to the
polarization beam splitter 41. The other one, an optical pulse P2,
goes along a long path and arrives at the polarization beam
splitter 41 after passing through the phase modulator 42 provided
in the long path. These optical pulses P1 and P2 are combined at
the polarization beam splitter 41 and then sent, as double pulses,
to the quantum transmitter 30 through the optical
multiplexer/demultiplexer 45 and the optical fiber transmission
line 300.
[0075] In the quantum transmitter 30, the double pulses P1 and P2,
having arrived through the optical fiber transmission line 300 and
then the optical multiplexer/demultiplexer 35, are each further
split at the polarization beam splitter 31, resulting in four
pulses (i.e., quartet pulses) including clockwise double pulses
P1.sub.CW and P2.sub.CW and counterclockwise double pulses
P1.sub.CCW and P2.sub.CCW. The clockwise double pulses and the
counterclockwise double pulses pass through the phase modulator 32
in the reverse directions to each other, and each pulse enters a
PBS port on the other side, different from the port from which the
pulse has come out.
[0076] The phase modulator 32 performs a phase modulation on the
pulse P2.sub.CW, the second-coming one of the clockwise double
pulses, relatively to the first-coming pulse P1.sub.CW. In addition
to this, the phase modulator 32 also gives a phase difference of
.pi. between the clockwise double pulses and the counterclockwise
double pulses. The quartet pulses phase-modulated as required in
this manner are combined at the PBS 31, returning again to the
double pulses. As described above, since the second pulse only has
been phase-modulated based on transmission information, the output
double pulses are denoted by P1 and P2*.sup.a. At this time, when
the pulses are outputted, the polarizations have been rotated by 90
degrees from the polarizations when the pulses were inputted into
the PBS loop. Therefore, as a result, an effect similar to that of
a Faraday mirror can be obtained.
[0077] Since the polarization states of the optical pulses P1 and
P2*.sup.a received from the quantum transmitter 30 have been
rotated by 90 degrees, the polarization beam splitter 41 of the
quantum receiver 40 leads each of these received optical pulses to
the other path different from the one used when the pulse was sent
to the quantum transmitter 30. Specifically, the received optical
pulse P1 is allowed along the long path and subjected at the phase
modulator 42 to a phase modulation according to a random number of
a random-number string RND2 from the random number generator 43,
and the phase-modulated optical pulse p1*.sup.b arrives at the
optical coupler 46. On the other hand, the optical pulse P2*.sup.a
goes along the short path, which is different from the path used
when the optical pulse P2 was sent to the quantum transmitter 30,
and similarly arrives at the optical coupler 46.
[0078] In this manner, the optical pulse P2*.sup.a, phase-modulated
in the quantum transmitter 30, and the optical pulse P1*.sup.b,
phase-modulated in the quantum receiver 40, interfere with each
other, and the result of this interference is detected by the
photon detector section 48, which is driven in a Geiger mode in
accordance with a clock signal supplied from the synchronization
section 44. The photon detector section 48 outputs a detection
signal to the key generator 210. Incidentally, the synchronization
sections 34 and 44 accomplish bit synchronization for the time of
key generation, as well as frame synchronization, by using
classical synchronization signals. Photon transmission is carried
out by the quantum transmitter 30 and the quantum receiver 40 as
described above.
[0079] FIG. 6A is a schematic diagram showing an example of the
optical switch, and FIG. 6B is a schematic diagram showing another
example of the optical switch. For each of the optical switches 271
and 272, a mechanical optical switch as shown in FIG. 6A can be
used. This switch shown in FIG. 6A is of a 1.times.2 type and
connects a port 0 fixed to a fixing jig 2703 to a port 1 or 2 by
using an electromagnet 2701 or 2702, respectively. Although the
switching speed is low, this switch has the characteristics of
small loss and excellent stability after switching takes place.
[0080] Moreover, for each of the optical switches 271 and 272, an
optical switch of a Mach-Zehnder type as shown in FIG. 6B can also
be used. This switch shown in FIG. 6B splits a light stream
inputted from a port 0 by using a directional coupler 2704 and
controls the phase of each light pulse stream by changing the
refractive index of a control portion 2705 or 2706. When a phase
difference is set to 0, an output comes out of a port 1 as a result
of interference at a directional coupler 2707. When a phase
difference is set to .pi./2, an output comes out of a port 2. In
the case of utilizing the electro-optic effect typically obtained
by PLZT {(Pb,La)(ZrTi)O.sub.3}, although a high switching speed of
nanosecond (ns) order can be achieved, the loss is large in
comparison with a mechanical switch.
[0081] For quantum channels, greater importance is placed on small
loss and stability than on switching speed. Therefore, it is
desirable to use a mechanical optical switch as shown in FIG. 6A
for the optical switch 271. For classical channels, greater
importance is placed on switching speed. Therefore, it is desirable
to use a Mach-Zehnder optical switch as shown in FIG. 6B for the
optical switch 272.
[0082] Note that for the quantum key distribution technique, any
one of a plug and play scheme, one-way scheme, and differential
phase-shift scheme may be used. The quantum key distribution
protocol is not limited to the BB84 protocol but may be the B92
protocol or the E91 protocol. The present invention will not be
restricted to the foregoing.
1.5) Second Embodiment
[0083] FIG. 7 is a block diagram showing a schematic configuration
of a secret communications system according to a second embodiment
of the present invention. Here, remote nodes 100-1 to 100-N are
individually connected to a center node 201 through optical fiber
transmission lines 300-1 to 300-N, respectively. The configuration
of each remote node 100-i is substantially the same as that in the
first embodiment shown in FIG. 4, and therefore description thereof
will be omitted.
[0084] The center node 201 includes a key generator (Key Gen.) 210,
a quantum receiver (Q_Rx) 220, optical multiplexers/demultiplexers
230-1 to 230-N, classical transceivers (CTRx) 241-1 to 241-N, an
encoder/decoder (Encode/Decode) 250, a key memory 260, an optical
switch 271, an electrical switch 273, and switch controllers 280
and 291. As in the first embodiment, a quantum channel and a
classical channel use mutually different wavelengths. Wavelength
multiplexing and demultiplexing are performed by the optical
multiplexers/demultiplexers 130-1 to 130-N and 230-1 to 230-N.
[0085] The point different from the first embodiment is that the
classical channels are switched not by an optical switch but by an
electrical switch 273. The electrical switch 273 selects a
classical transceiver 241-i, whereby quantum key generation and
sharing based on the result of photon transmission and also
encrypted communication using the quantum key are carried out
between the selected classical transceiver 241-i and the
corresponding classical transceiver 140-i of the remote node 100-i
through the corresponding classical channel. The quantum key
generation is carried out between the key generators 110-i and 210
through the classical channel, and the generated key Ki is stored
in each of the key memories 160-i and 260. At the time of an
encrypted communication, for example, one-time pad encryption is
performed by using the key Ki generated for each remote node.
[0086] In the case of the one-time pad encryption, a key is
discarded each time an encrypted communication (encoding and
decoding) is carried out. Accordingly, the key in the key memory
260 is consumed depending on the amount of an encrypted
communication. Therefore, the switch controller 280 monitors the
amounts of the keys in the key memory 260, generated and shared
with the remote nodes respectively, and performs switching control
on the optical switch 271, based on the monitored amounts of the
keys.
[0087] Switching between the quantum key generation and sharing and
the encrypted communication is performed by each classical
transceiver 241-i. Control on the electrical switch 273 to switch
nodes to connect is performed by the switch controller 291. The
switching control of classical channels is performed independently
of the switching control of quantum channels, as shown in the time
chart of FIG. 3(b).
[0088] As described above, the switching of nodes to connect
through a quantum channel and the switching of nodes to connect
through a classical channel are separately handled by the optical
switch 271 and the electrical switch 273 and thus performed
independently, whereby it is possible to realize efficient photon
transmission, quantum key generation and sharing, and encrypted
communication in the point-to-multipoint connection. In addition,
by multiplexing and transmitting the quantum and classical channels
over a single fiber, it is possible to construct a system at a low
cost for fiber laying.
Example II
[0089] Next, a specific example will be described in which the
above-described second embodiment is applied to a one-way quantum
key distribution (QKD) system.
[0090] FIG. 8A is a block diagram showing an example of a quantum
transmitter on Alice's side (remote-node side) in a one-way QKD
system. FIG. 8B is a block diagram showing an example of a quantum
receiver on Bob's side (center-node side) in the one-way QKD
system. The quantum transmitter 50 shown in FIG. 8A and the quantum
receiver 60 shown in FIG. 8B constitute a time-division pulse
interferometer by using asymmetric Mach-Zehnder interferometers
(AMZ) that are based on planar lightwave circuit (PLC) technology
(see Kimura, T., Nambu, Y., Hatanaka, T., Tomita, A., Kosaka, H.,
and Nakamura, K., "Single-photon Interference over 150 km
Transmission Using Silica-based Integrated-optic Interferometers
for Quantum Cryptography," Japanese Journal of Applied Physics
Letters, Vol. 43, No. 9A/B (2004), pp. L1217-L1219).
[0091] The quantum transmitter 50 includes a PLC-based asymmetric
Mach-Zehnder interferometer (AMZ (PLC)) 51, a phase modulator
(PM.sub.A) 52, a random number generator (Rnd.) 53, a
synchronization section (Sync) 54, an optical
multiplexer/demultiplexer 55, and a pulse light source 56. The
quantum transmitter 50 is connected to an optical fiber
transmission line 300. The quantum receiver 60 includes a PLC-based
asymmetric Mach-Zehnder interferometer (AMZ (PLC)) 61, a phase
modulator (PM.sub.B) 62, a random number generator (Rnd.) 63, a
synchronization section (Sync) 64, an optical
multiplexer/demultiplexer 65, and a photon detector section 68. The
quantum receiver 60 is connected to the optical fiber transmission
line 300.
[0092] In the quantum transmitter 50, an optical pulse outputted
from the pulse light source 56 is phase-modulated by the phase
modulator 52 in accordance with random numbers of two random-number
strings RND0 and RND1 supplied from the random number generator 53,
and is split into two time-divided optical pulses (preceding and
following pulses) by the asymmetric Mach-Zehnder interferometer 51.
In the quantum receiver 60, one of the preceding and following
pulses is phase-modulated by the phase modulator 62 in accordance
with a random number of a random-number string RND2 supplied from
the random number generator 63. These preceding and following
optical pulses enter the asymmetric Mach-Zehnder interferometer 61,
where the following one of the split preceding pulses interferes
with the preceding one of the split following pulses. The result of
this interference is detected by the photon detector section 68.
Note that the synchronization sections 54 and 64 transmit
synchronization signals to each other through a classical channel,
whereby bit synchronization for the time of key generation as well
as frame synchronization are accomplished.
1.6) Third Embodiment
[0093] FIG. 9 is a block diagram showing a schematic configuration
of a secret communications system according to a third embodiment
of the present invention. Here, remote nodes 101-1 to 101-N are
individually connected to a center node 202 through optical fiber
transmission lines 300-1 to 300-N, respectively. Note that the
blocks having equivalent or similar functions as those in the first
embodiment shown in FIG. 4 are given the same reference numerals as
in FIG. 4, and description thereof will be omitted.
[0094] According to the present embodiment, each remote node 101-i
is provided with a synchronization section 104-i, but not in the
quantum transmitter 121-i. The center node 202 is provided with a
synchronization section 204, but not in the quantum receiver 221.
The synchronization sections 104-i and 204 according to the present
embodiment are connected to each other all the time through a
classical channel, not via an optical switch. Therefore, efficient
quantum key generation and sharing can be realized because it is
not necessary to follow such a procedure, as in the first and
second embodiments, that after switching control is performed on
the switches, synchronization is established, and then a
cryptographic key is generated. In addition, the configurations of
the quantum units can be simplified because there is no need to
provide synchronization sections in the quantum transmitter and
quantum receiver.
[0095] The third embodiment of the present invention can be applied
to any one of a plug and play QKD system and a one-way QKD
system.
[0096] FIG. 10A is a block diagram showing another example of the
quantum transmitter on Alice's side (remote-node side) in a plug
and play QKD system, and FIG. 10B is a block diagram showing
another example of the quantum receiver on Bob's side (center-node
side) in the plug and play QKD system. The quantum transmitter 30
shown in FIG. 10A and the quantum receiver 40 shown in FIG. 10B are
of the alternative-shifted phase modulation, plug and play type and
have configurations similar to the example shown in FIGS. 5A and
5B, respectively, except the synchronization sections. Therefore,
the same reference numerals as in FIGS. 5A and 5B are used in FIGS.
10A and 10B, and description thereof will be omitted.
[0097] FIG. 11A is a block diagram showing another example of the
quantum transmitter on Alice's side (remote-node side) in a one-way
QKD system, and FIG. 11B is a block diagram showing another example
of the quantum receiver on Bob's side (center-node side) in the
one-way QKD system. The quantum transmitter 50 shown in FIG. 11A
and the quantum receiver 60 shown in FIG. 11B constitute a
time-division pulse interferometer by using asymmetric Mach-Zehnder
interferometers (AMZ) that are based on planar lightwave circuit
(PLC) technology. In this case as well, the quantum transmitter 50
and the quantum receiver 60 have configurations similar to those
shown in FIGS. 8A and 8B, respectively, except the synchronization
sections. Therefore, the same reference numerals as in FIGS. 8A and
8B are used in FIGS. 11A and 11B, and description thereof will be
omitted.
2. Second Mode
[0098] In the above-described first mode, description is given of
the case of point-to-multipoint connections between the center node
and the multiple nodes. However, according to the present
invention, it is possible to realize multipoint-to-multipoint
connections by allowing all the remote nodes and the center node to
share the same cryptographic key.
[0099] FIG. 12 is a block diagram showing a schematic configuration
of a secret communications system according to a second mode of the
present invention. Here, remote nodes 103-1 to 103-N are
individually connected to a center node 203 through respective
optical fiber transmission lines, and encrypted communications are
carried out between the remote nodes 103-1 to 103-N via the center
node 203.
[0100] Note that, although FIG. 12 shows only a key memory section
163-i in each remote node 103-i and only a classical-channel switch
section 273 and a key memory section 263 in the center node 203,
the other components are substantially the same as those in the
first mode shown in FIG. 1. Therefore, illustration and description
thereof will be omitted.
[0101] The key memory section 163-i of each remote node 103-i
includes a key memory 165-i for storing a cryptographic key Ki used
for individual communication with the center node 203, and a key
memory 164-i for storing a common cryptographic key K.sub.mult used
for multipoint-to-multipoint communication. Similarly, the key
memory section 263 of the center node 203 includes a key memory 265
for storing the cryptographic keys K1 to KN used for individual
communication with the respective remote nodes, and a key memory
264 for storing the common cryptographic key K.sub.mult used for
multipoint-to-multipoint communication.
[0102] The cryptographic keys K1 to KN used for individual
communication between the respective remote nodes and the center
node are generated and shared as described in the first mode.
[0103] The common cryptographic key K.sub.mult for
multipoint-to-multipoint communication is generated and set as
follows. First, the controller (at 105 in FIG. 1) of the center
node 203 generates the common cryptographic key K.sub.mult based on
part of the cryptographic keys K1 to KN for individual
communication stored in the key memory 265, or part of at least one
of the cryptographic keys K1 to KN, and then stores the generated
common cryptographic key K.sub.mult in the key memory 264. Next,
the controller encrypts, based on the one-time pad (Vernam cipher)
scheme, the common cryptographic key K.sub.mult stored in the key
memory 264 by using each cryptographic key Ki unique to the
corresponding remote node and sends the encrypted key to each
remote node. Thus, all the remote nodes and the center node can
share the common cryptographic key K.sub.mult. Although the
physical topology is one to N, multipoint-to-multipoint encrypted
communication can be realized.
[0104] As described above, the switching of nodes to connect
through a quantum channel and the switching of nodes to connect
through a classical channel are separately handled and thereby can
be performed independently. Thus, it is possible not only to
realize efficient point-to-multipoint photon transmission and
quantum key generation, but also to realize
multipoint-to-multipoint quantum key sharing and encrypted
communication. In addition, by multiplexing and transmitting the
quantum and classical channels over a single fiber, it is possible
to construct a system at a low cost for fiber laying.
2.1) Fourth Embodiment
[0105] FIG. 13 is a block diagram showing a schematic configuration
of a secret communications system according to a fourth embodiment
of the present invention. Here, remote nodes 100-1 to 100-N are
individually connected to a center node 200 through optical fiber
transmission lines 300-1 to 300-N, respectively. Note that the
blocks having equivalent or similar functions as those in the first
embodiment shown in FIG. 4 are given the same reference numerals as
in FIG. 4, and description thereof will be omitted.
[0106] In the configuration according to the present embodiment,
each key memory 160-i in the first embodiment shown in FIG. 4 is
replaced by a key memory section 163-i. Specifically, the key
memory section 163-i includes a key memory 165-i for storing a
cryptographic key Ki used for individual communication with the
center node 200, and a key memory 164-i for string a common
cryptographic key K.sub.mult used for multipoint-to-multipoint
communication. Similarly, a key memory section 263 of the center
node 200 includes a key memory 265 for storing the cryptographic
keys K1 to KN used for individual communication with the respective
remote nodes, and a key memory 264 for storing the common
cryptographic key K.sub.mult used for multipoint-to-multipoint
communication.
[0107] The quantum transmitter 120-i and the quantum receiver 220
may be of the plug and play type shown in FIGS. 5A and 5B or may be
of the one-way type shown in FIGS. 8A and 8B. For the
quantum-channel optical switch 271, since greater importance is
placed on small loss and stability than on switching speed, a
mechanical optical switch as shown in FIG. 6A is used in the
present embodiment. For the classical-channel optical switch 272,
since greater importance is placed on switching speed, a
Mach-Zehnder optical switch as shown in FIG. 6B is used in the
present embodiment.
[0108] A quantum cryptographic key Ki unique to each remote node
100-i is generated and shared between each remote node 100-i and
the center node 200 as described already. The center node 200
generates the common cryptographic key K.sub.mult from part of
these keys K1-KN and stores it in the key memory 264. Next, the
center node 200 encrypts, based on the one-time pad (Vernam cipher)
scheme, the common cryptographic key K.sub.mult stored in the key
memory 264 by using each unique cryptographic key Ki and sends the
encrypted key to each remote node 100-i. Thus, all the remote nodes
100-1 to 100-N and the center node 200 can share the common
cryptographic key K.sub.mult. Accordingly, although the physical
topology is one to N, it is possible to realize
multipoint-to-multipoint encrypted communication.
[0109] Note that for the quantum key distribution technique, any of
a plug and play scheme, one-way scheme, and differential
phase-shift scheme may be used. The quantum key distribution
protocol is not limited to the BB84 protocol but may be the B92
protocol or the E91 protocol. The present invention will not be
restricted to the foregoing.
[0110] The present invention can be utilized for
point-to-multipoint, as well as multipoint-to-multipoint, secret
information communication using common-cryptographic-key
distribution technology typified by quantum key distribution
(QKD).
* * * * *